Radio communication system, base station, mobile station, communication control method, and computer readable medium

ABSTRACT

A first base station ( 1 ) is configured to send, to a second base station ( 2 ), first configuration information that is necessary to establish a data bearer and a data radio bearer in the second base station ( 2 ) for a C/U-plane split scenario (S 105 ). Further, the first base station ( 1 ) is configured to keep the first configuration information in the first base station ( 1 ) even after the data bearer and the data radio bearer are established in the second base station ( 2 ) (S 109 ). It is thus, for example, possible to contribute to a reduction in a path switch delay when a UE moves between cells in the C/U-plane split scenario.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a National Stage Entry of International ApplicationNo. PCT/JP2013/004746, filed Aug.6, 2013, which claims priority fromJapanese Patent Application No.2012-288209, filed Dec.28, 2012. Theentire contents of the above-referenced applications are expresslyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a radio communication system, and moreparticularly, to network architecture in a small cell enhancementscenario.

BACKGROUND ART

In the Long Term Evolution (LTE) Release 12 according to the 3rdGeneration Partnership Project (3GPP), “local area enhancement” or“small cell enhancement” for accommodation of a large amount of localtraffic, improvement in throughput, and efficient use of ahigh-frequency band has become one of the subjects for discussion (seeNon-patent literature 1). In the local area enhancement or the smallcell enhancement, a low-power node (LPN) that forms a small cell isused.

Further, a C/U-plane split scenario has been proposed regarding thesmall cell enhancement. In the C/U-plane split, a macro cell provides acontrol plane (e.g., Radio Resource Control (RRC) connection, andNon-Access Stratum (NAS) message transfer) for a mobile station (UserEquipment (UE)) and a small cell provides a user plane for the UE. Inone specific example of the C/U-plane split scenario, for the Controlplane (C-plane), the macro cell can keep a good connection with the UEby a wide coverage using a low frequency band and support mobility ofthe UE. Meanwhile, for the user plane (U-plane), the small cell canprovide a local high throughput for the UE by using a wide bandwidth ina high frequency band.

In the C/U-plane split scenario, a case in which a small cell does notrequire transmission of existing cell specific signals/channels (e.g.,Primary Synchronization Signal (PSS), Secondary Synchronization Signal(SSS), Cell-specific Reference Signal (CRS), Master Information Block(MIB), and System Information Block (SIB)) is also assumed. Such a newsmall cell may be referred to as a phantom cell. Further, a base station(eNB) or an LPN that provides a small cell may be referred to as aPhantom eNodeB (PhNB).

CITATION LIST Non Patent Literature

-   [Non-Patent Literature 1] 3GPP RWS-120010, NTT DOCOMO,    “Requirements, Candidate Solutions & Technology Roadmap for LTE    Rel-12 Onward”, 3GPP TSG RAN Workshop on Rel-12 and Onwards    Ljubljana, Slovenia, 11-12 Jun. 2012

SUMMARY OF INVENTION Technical Problem

As described above, the C/U-plane split scenario in which the C-plane isprovided for UEs in a cell controlled by the MeNB and the U-plane isprovided for the UEs in a cell controlled by the LPN has been proposed.In the following description, a cell that provides the C-Plane in theC/U-plane split scenario is referred to as a primary cell (PCell) and acell that provides the U-Plane in the C/U-plane split scenario isreferred to as a secondary cell (SCell).

The present inventors have studied about an inter-cell movement of a UEin the C/U-plane split scenario and have found various problemstherewith. Consider a case in which a UE moves within a coverage of oneMeNB cell (PCell) and accordingly moves outside of a first LPN cell(SCell) with which the UE establishes a data radio bearer (DRB) in theC/U-plane split scenario. In this case, first and second mobilityscenarios that will be described below may be considered.

In the first mobility scenario, the first LPN cell is sparsely deployedso that the first LPN cell does not overlap other LPN cells, and thusthe UE changes the SCell (i.e., DRB that is established in the SCell)from the first LPN cell to the MeNB cell. If a normal S1 handoverprocedure is applied to the UE movement in the first mobility scenario,wasted signaling occurs, which increases a path switch delay (i.e., databearer switching delay time). This is because, since the C-Plane isestablished in the MeNB cell, there is no need to change the C-Plane inthe first handover scenario. Accordingly, when the normal handoverprocedure is used, signaling to change the C-Plane from a source cell toa target cell is wasted. Therefore, special considerations must be takenin the C/U-plane split scenario.

In the second mobility scenario, the first LPN cell and a second

LPN cell are closely deployed so that the first LPN cell partiallyoverlaps the second LPN cell. Therefore, the UE changes the S Cell(i.e., DRB established in the SCell) from the first LPN cell to thesecond LPN cell. Similar to the first mobility scenario, in the secondmobility scenario, the problem regarding the path switch delay occurs.

Accordingly, one object of the present invention is to provide a radiocommunication system, a base station, a mobile station, a communicationcontrol method, and a program that contribute to a reduction in a pathswitch delay when a UE moves between cells in the C/U-plane splitscenario.

Solution to Problem

In a first aspect, a radio communication system includes first andsecond base stations, a core network, and a mobile station. The corenetwork includes a mobility management apparatus and a data transferapparatus. The first base station operates a first cell and the secondbase station operates a second cell. The first base station isconfigured to establish a first signaling bearer with the mobilitymanagement apparatus, establish a second signaling bearer with thesecond base station, and establish a signaling radio bearer with themobile station in the first cell. The second base station is configuredto establish the second signaling bearer with the first base station,establish a data bearer with the data transfer apparatus, and establisha data radio bearer with the mobile station in the second cell. Thefirst base station is further configured to send, to the second basestation via the second signaling bearer, first configuration informationthat is necessary to establish the data bearer and the data radio bearerin the second base station. The first base station is further configuredto keep the first configuration information in the first base stationeven after the data bearer and the data radio bearer are established inthe second base station.

In a second aspect, a first base station includes a radio communicationunit that operates a first cell, and a controller. The controller isconfigured to perform control to establish a first signaling bearer witha mobility management apparatus in a core network, establish a secondsignaling bearer with a second base station that operates a second cell,and establish a signaling radio bearer with a mobile station in thefirst cell. The controller is further configured to send, to the secondbase station via the second signaling bearer, first configurationinformation that is necessary to establish a data bearer and a dataradio bearer in the second base station. The controller is furtherconfigured to keep the first configuration information even after thedata bearer and the data radio bearer are established in the second basestation. The data bearer is established between the second base stationand a data transfer apparatus in the core network. The data radio beareris established between the second base station and the mobile station inthe second cell.

In a third aspect, a mobile station is used in combination with theradio communication system according to the above first aspect, andincludes a radio communication unit and a controller. The controller isconfigured to control the radio communication unit to receiveconfiguration information regarding the data radio bearer from the firstbase station and receive or transmit user data using the second cell.

In a fourth aspect, a communication control method, in a first basestation that operates a first cell, includes:

(a) performing control to establish a first signaling bearer with amobility management apparatus in a core network, establish a secondsignaling bearer with a second base station that operates a second cell,and establish a signaling radio bearer with a mobile station in thefirst cell;

(b) sending, to the second base station via the second signaling bearer,first configuration information that is necessary to establish a databearer and a data radio bearer in the second base station, wherein thedata bearer is established between the second base station and a datatransfer apparatus in the core network, and the data radio bearer isestablished between the second base station and the mobile station inthe second cell; and

(c) keeping the first configuration information in the first basestation even after the data radio bearer and the data bearer areestablished in the second base station.

In a fifth aspect, a program includes instructions for causing acomputer to perform the communication control method according to theabove fourth aspect.

Advantageous Effects of Invention

According to the above aspects, it is possible to provide a radiocommunication system, a base station, a mobile station, a communicationcontrol method, and a program which contribute to a reduction in a pathswitch delay when a UE moves between cells in the C/U-plane splitscenario.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration example of a radiocommunication system (e.g., LTE system) according to a first embodiment;

FIG. 2 is a diagram showing one example of bearer architecture in theradio communication system according to the first embodiment;

FIG. 3 is a diagram showing another example of the bearer architecturein the radio communication system according to the first embodiment;

FIG. 4 is a diagram showing a configuration example of a first basestation (e.g., MeNB) according to the first embodiment;

FIG. 5 is a diagram showing a configuration example of a second basestation (e.g., LPN) according to the first embodiment;

FIG. 6 is a diagram showing a configuration example of a mobile station(e.g., UE) according to the first embodiment;

FIG. 7 is a diagram showing a configuration example of a mobilitymanagement apparatus (e.g., MME) according to the first embodiment;

FIG. 8 is a diagram showing a configuration example of a data transferapparatus (e.g., S-GW) according to the first embodiment;

FIG. 9 is a sequence diagram showing one example of a communicationcontrol method according to the first embodiment;

FIG. 10 is a diagram showing a configuration example of a radiocommunication system (e.g., LTE system) according to a secondembodiment;

FIG. 11 is a sequence diagram showing a bearer switch procedureaccording to a movement of a mobile station according to the secondembodiment;

FIG. 12 is a flowchart showing an operation example of a first basestation (e.g., MeNB) according to the second embodiment;

FIG. 13 is a flowchart showing an operation example of a second basestation (e.g., LPN) according to the second embodiment;

FIG. 14 is a flowchart showing an operation example of a mobile station(e.g., UE) according to the second embodiment;

FIG. 15 is a flowchart showing an operation example of a mobilitymanagement apparatus (e.g., MME) according to the second embodiment;

FIG. 16 is a flowchart showing an operation example of a data transferapparatus (e.g., S-GW) according to the second embodiment;

FIG. 17 is a diagram showing a configuration example of a radiocommunication system (e.g., LTE system) according to a third embodiment;

FIG. 18 is a sequence diagram showing a bearer switch procedureaccording to a movement of a mobile station according to the thirdembodiment;

FIG. 19 is a flowchart showing an operation example of a first basestation (e.g., MeNB) according to the third embodiment;

FIG. 20 is a diagram showing a configuration example of a radiocommunication system (e.g., LTE system) according to a fourthembodiment;

FIG. 21 is a sequence diagram showing a bearer switch procedureaccording to a movement of a mobile station according to the fourthembodiment;

FIG. 22 is a flowchart showing an operation example of a first basestation (e.g., MeNB) according to the fourth embodiment; and

FIG. 23 is a flowchart showing an operation example of a second basestation (e.g., LPN) according to the fourth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, with reference to the drawings, specific embodiments willbe described in detail. Throughout the drawings, identical orcorresponding components are denoted by the same reference symbols, andoverlapping descriptions will be omitted as appropriate for the sake ofclarification of description.

First Embodiment

FIG. 1 shows a configuration example of a radio communication systemaccording to the embodiment. The radio communication system according tothe embodiment includes a first base station 1, a second base station 2,a mobile station 4, and a core network 5. The base stations 1 and 2operate a first cell 10 and a second cell 20, respectively. The corenetwork 5 includes a mobility management apparatus 6 and a data transferapparatus 7. In the following description, for the sake ofsimplification of the description, a case in which the radiocommunication system according to the embodiment is an LTE system willbe described as an example. Accordingly, the first base station 1corresponds to an MeNB, the second base station 2 corresponds to an LPN,the mobile station 4 corresponds to a UE, the core network 5 correspondsto an Evolved Packet Core (EPC), the mobility management apparatus 6corresponds to a Mobility Management Entity (MME), and the data transferapparatus 7 corresponds to a Serving Gateway (S-GW).

The radio communication system according to the embodiment applies theC/U-plane split to the cells 10 and 20. That is, the LPN 2 providesU-plane services for the UE 4 in the cell 20. In other words, the LPN 2establishes a data radio bearer (DRB) with the UE 4 and transfers userdata of the UE 4. The MeNB 1 provides C-Plane services in the cell 10for the UE 4 which establishes the DRB with the LPN 2. In other words,the MeNB 1 establishes a signaling radio bearer (SRB) with the UE 4 andprovides RRC signaling, for example, to establish and modify the DRB inthe cell 20 of the LPN 2, and NAS message transfer between the EPC 5 andthe UE 4. The MeNB 1 may transmit, on a downlink channel (e.g., PhysicalBroadcast Channel (PBCH) or Physical Downlink Shared Channel (PDSCH)) ofthe cell 10, master information (e.g., system bandwidth, and the numberof transmission antennas) and system information (e.g., parametersregarding the DRB in the cell 20) regarding the cell 20 of the LPN 2.

The MeNB 1 may not provide all the C-plane services regarding the UE 4.For example, the LPN 2 may control a layer 1 (physical layer) and alayer 2 (Media Access Control (MAC) sublayer and Radio Link Control(RLC) sublayer) regarding the data radio bearer that is established forthe LPN 2. Specifically, the LPN 2 may receive layer 1/layer 2 controlsignals (e.g., Hybrid Automatic Repeat Request (H-ARQ) ACK, ChannelQuality Indicator (CQI), Precoding Matrix Indicator (PMI), and RankIndicator (RI)) using an uplink control channel (e.g., Physical UplinkControl Channel (PUCCH)) or an uplink data channel (e.g., PhysicalUplink Shared Channel (PUSCH)). The LPN 2 may transmit downlinkscheduling information, ACK/NACK for uplink transmission and the like tothe UE 4 using a downlink control channel (e.g., Physical DownlinkControl Channel (PDCCH)).

The EPC 5 is a network that is generally managed by an operator thatprovides mobile communication services. The EPC 5 has control plane(C-plane) functions including mobility management (e.g., locationregistration and location update) and bearer management (e.g., bearerestablishment, bearer modification, and bearer release) of the UE 4, anduser plane (U-plane) functions including transferring user data of theUE 4 between the MeNB 1 and an external network (not shown) and betweenthe LPN 2 and the external network. The MME 6 contributes to the C-planefunctions in the EPC. The S-GW 7 contributes to the U-plane functions inthe EPC. The S-GW 7 is arranged at a boundary between the EPC 5 and aradio access network (RAN) including the MeNB 1 and the LPN 2.

In the following description, with reference to FIGS. 2 and 3, thebearer architecture according to this embodiment will be described. FIG.2 shows a first example of the bearer architecture related to the userdata transfer in the cell 20. The radio bearer has already beendescribed above. That is, the MeNB 1 establishes the SRB with the UE 4,and provides, in the cell 10, C-plane services including RRC signaling,for example, to establish and modify the DRB on the cell 20 and NASmessage transfer between the EPC 5 and the UE 4. Meanwhile, the LPN 2establishes the DRB with the UE 4 and transmits and receives the userdata of the UE 4 in the cell 20.

Next, bearers between the EPC 5 and the MeNB 1 and between the EPC 5 andthe LPN 2 will be described. A signaling bearer (i.e., S1 signalingbearer using an S1-MME interface) with the EPC 5 is established betweenthe MME 6 and the MeNB 1. The MeNB 1 establishes the S1 signaling bearerwith the MME 6 and sends and receives S1 Application Protocol (S1-AP)messages to and from the MME 6. Meanwhile, a data bearer (i.e., S1bearer using an S1-U interface) with the EPC 5 is established betweenthe S-GW 7 and the LPN 2. The LPN 2 establishes the S1 bearer with theS-GW 7 and sends and receives user data of the UE 4 to and from the S-GW7.

Further, the MeNB 1 establishes a signaling bearer with the LPN 2. Thesignaling bearer between the MeNB 1 and the LPN 2 is established using,for example, an X2 interface. The X2 interface is an interface betweeneNBs. A case in which the LPN 2 is defined as a new node and a newinterface different from the X2 interface is defined between the eNB andthe LPN may be considered. In this case, the signaling bearer betweenthe MeNB 1 and the LPN 2 may be established using this new interface. Inthis specification, this new interface is provisionally referred to asan X3 interface. The MeNB 1 is configured to send, to the LPN 2 via anX2/X3 signaling bearer, bearer configuration information (hereinafterreferred to as E-UTRAN Radio Access Bearer (E-RAB) configurationinformation) that is necessary to establish the S1 bearer with the S-GW7 and the DRB with the UE 4 in the LPN 2. The E-RAB is a radio accessbearer including the DRB and the S1 bearer.

According to the bearer architecture shown in FIG. 2, the LPN 2 does notrequire the S1 signaling bearer with the MME 6 and can set up the DRBand the S1 bearer based on E-RAB configuration information supplied fromthe MeNB 1. In addition, in the above-mentioned bearer architecture, atermination point of the S1 bearer (S1-U bearer) is different from atermination point of the S1 signaling bearer. That is, the LPN 2, notthe MeNB 1, terminates the S1 bearer. That is, in the architecture shownin FIG. 2, the C/U planes are separated not only with regard to thesignaling in the RAN but also with regard to interfaces between the EPC5 and the RAN. As a result of this, the MeNB 1 is only required toperform signaling to establish the S1 bearer and the DRB necessary forthe UE 4 to transmit and receive user data via the cell 20 and the LPN2. In other words, in one example, the MeNB 1 needs not to terminate theS1 bearer (i.e., GPRS Tunneling Protocol (GTP) tunnel) for thecommunication of the UE 4 via the cell 20, and also needs not to performforwarding of user data packets between the S1 bearer and the DRB. Theseprocessing are performed by the LPN 2. Accordingly, in one example, itis possible to reduce the processing load on the MeNB 1.

The S1 bearer is a GTP tunnel and the user data (data packet) isencapsulated in GTP tunnel packets to be transferred between the S-GW 7and the LPN 2. For example, the GTP tunnel packets that encapsulatedownlink user data arrive at the LPN 2 by being subjected to routing andforwarding by routers arranged between the S-GW 7 and the LPN 2.Accordingly, in the bearer architecture shown in FIG. 2, typically, theGTP tunnel packets are transferred without passing through the MeNB 1.In this case, the MeNB 1 need not carry out processing for terminatingthe S1 bearer and thus it is possible to reduce the processing load onthe MeNB 1. Further, since the GTP tunnel packets do not flow throughthe X2/X3 interface between the MeNB 1 and the LPN 2, performancerequirements on the capacity, the delay and the like of the X2/X3interface are relaxed. It is possible, for example, to use a non-opticalfiber line (e.g., wireless communication path) for the X2/X3 interface.

However, in some implementations, the GTP tunnel packets thatencapsulate the user data may be transferred between the S-GW 7 and theLPN 2 via the MeNB 1. In this case, the MeNB 1 may function as a router(e.g., Internet Protocol (IP) router) and may perform routing andforwarding of the GTP tunnel packets. The routing of the GTP tunnelpackets that pass through the MeNB 1 can be achieved by setting uprouting tables included in the S-GW 7, the LPN 2, and the MeNB 1.

FIG. 3 shows a second example of the bearer architecture. In the exampleshown in FIG. 3, the MeNB 1 performs routing and forwarding of the GTPtunnel packets. The MeNB 1 may have a proxy function to convert the IPaddresses of the GTP tunnel packets. Specifically, the MeNB 1 and theLPN 2 set up a tunnel 80 (e.g., GTP Tunnel) via the X2/X3 interface. TheMeNB 1 further encapsulates the GTP tunnel packets, which encapsulatethe user data on the S1 bearer between the S-GW 7 and the LPN 2, andforwards the encapsulated GTP tunnel packets using the tunnel 80. Thetunnel 80 may be omitted. That is, the MeNB 1 may directly forward theGTP tunnel packets without performing further encapsulation of the GTPtunnel packets.

One notable point in the example shown in FIG. 3 is that the MeNB 1 neednot terminate the S1 bearer. The MeNB 1 is only required to operate as arouter that forwards the GTP tunnel packets and need not performdecapsulation processing to retrieve user packets. Accordingly, anincreased processing load on the MeNB 1 which is due to the GTP tunneltermination does not occur.

Another notable point in the example shown in FIG. 3 is that the MeNB 1can monitor the GTP tunnel packets. The MeNB 1 can monitor, for example,the traffic amount of the GTP tunnel packets to be transferred. Bymonitoring the traffic amount of the GTP tunnel packets, the MeNB 1 canautonomously estimate the load on the cell 20 or the load on the LPN 2.Accordingly, the MeNB 1 according to the embodiment can determinedeactivation of the cell 20 or the E-RAB that passes through the LPN 2,based on the traffic amount of the GTP tunnel packets monitored by theMeNB 1.

Next, the configurations and the operations of the apparatuses accordingto the embodiment will be described further in detail. The MeNB 1according to the embodiment is configured to hold the E-RABconfiguration information received from the MME 6 during an initial databearer establishment procedure for establishing the DRB and the S1bearer in the LPN 2, without discarding (releasing) the E-RABconfiguration information. In other words, the MeNB 1 is configured tokeep the E-RAB configuration information in the MeNB 1 even after theestablishment of the DRB and the S1 bearer in the LPN 2 based on theE-RAB configuration information. In one example, when the endpoints(termination points) of the DRB and the S1 bearer for the UE 4 arechanged from the LPN 2 to another base station (e.g., another eNB oranother LPN), the MeNB 1 may send the E-RAB configuration information,which has been kept in the MeNB 1, to another base station withoutre-sending a request for establishing the S1 bearer (or S1 handoverrequest) to the MME 6. In another example, when the endpoints(termination points) of the DRB and the S1 bearer for the UE 4 arechanged from the LPN 2 to the MeNB 1, the MeNB 1 may establish the S1bearer and the DRB in the MeNB 1 by re-using the E-RAB configurationinformation, which has been kept in the MeNB 1, without re-sending arequest for establishing the S1 bearer (or S1 handover request) to theMME 6. When the DRB for the UE 4 is established in the MeNB 1, this DRBmay be configured on the cell 10 or may be configured on a cell(secondary cell) of the MeNB 1 different from the cell 10.

As already stated above, the radio communication system according tothis embodiment employs the C/U Split architecture. Therefore, the MeNB1 is not only in charge of the C-Plane of the MeNB 1 but also in chargeof the C-Plane of the cell 20 of the LPN 2 arranged in the cell 10 ofthe MeNB 1. Accordingly, as long as the UE 4 moves within the cell 10,the MeNB 1 is in charge of the C-Plane of the S1 bearer and the DRB forthe UE 4 regardless of which one of the MeNB 1, the LPN 2, or anotherLPN in the cell 10 provides the S1 bearer and the DRB for the UE 4.Based on this point, the MeNB 1 according to the embodiment does notdiscard (release) the E-RAB configuration information received from theMME 6 during an initial data bearer establishment procedure and holdsthe E-RAB configuration information. When the endpoints of the S1 bearerand the DRB are changed to the MeNB 1 or another base station due to amovement of the UE 4 in the cell 10, the MeNB 1 re-uses the E-RABconfiguration information, which has been kept in the MeNB 1. It istherefore possible to omit at least a part of the handover processingperformed between the MeNB 1 and the MME 6 in this embodiment.Accordingly, this embodiment contributes to a reduction in the pathswitch delay (i.e., data bearer switching delay time) when the UE 4moves between cells in the C/U-plane split scenario.

In the following description, configuration examples of the MeNB 1, theLPN 2, the UE 4, the MME 6, and the S-GW 7 according to the embodimentwill be described. FIG. 4 is a block diagram showing a configurationexample of the MeNB 1. A radio communication unit 11 receives an uplinksignal transmitted from the UE 4 via an antenna. A reception dataprocessing unit 13 restores the received uplink signal. The resultantreceived data is transferred to another network node (e.g., the MME 6 orthe S-GW 7) via a communication unit 14. For example, uplink user datareceived from the UE 4 in the cell 10 is transferred to the S-GW 7.Further, NAS data among control data received from the UE 4 istransferred to the MME 6. Further, the reception data processing unit 13receives from a controller 15 the control data to be transmitted to theLPN 2 or the MME 6 and sends the control data to the LPN 2 or the MME 6via the communication unit 14.

A transmission data processing unit 12 acquires user data destined forthe UE 4 from the communication unit 14, and generates a transportchannel by performing error correction coding, rate matching,interleaving and the like on the user data. The transmission dataprocessing unit 12 then generates a transmission symbol sequence byadding control information to the data sequence of the transportchannel. The radio communication unit 11 generates a downlink signal byperforming processing such as carrier wave modulation based on thetransmission symbol sequence, frequency conversion, and signalamplification, and transmits the generated downlink signal to the UE 4.Furthermore, the transmission data processing unit 12 receives thecontrol data to be transmitted to the UE 4 from the controller 15 andtransmits the control data to the UE 4 via the radio communication unit11.

The controller 15 performs signaling with the MME 6, the LPN 2, and theUE 4 via the signaling bearers in order to enable the UE 4 to receive ortransmit the user data through the cell 20 operated by the LPN 2.Specifically, the controller 15 sends an establishment request of the S1bearer or the E-RAB to the MME 6 via the S1 signaling bearer. Thecontroller 15 sends, to the LPN 2 via the X2/X3 signaling bearer, E-RABconfiguration information that is necessary to establish the S1 bearerand the DRB in the LPN 2. The controller 15 transmits, to the UE 4 viathe SRB in the cell 10, the DRB configuration information that isnecessary to establish the DRB on the cell 20 in the UE 4.

Furthermore, the controller 15 is configured to keep the E-RABconfiguration information in the MeNB 1 even after the establishment ofthe S1 bearer and the DRB in the LPN 2 based on the E-RAB configurationinformation. In one example, when the endpoints (termination points) ofthe DRB and the S1 bearer for the UE 4 are changed from the LPN 2 toanother base station (e.g., another eNB or another LPN), the controller15 may send the E-RAB configuration information, which has been kept inthe MeNB 1, to another base station without re-sending the request forestablishing the S1 bearer (or S1 handover request) to the MME 6. Inanother example, when the endpoints (termination points) of the DRB andthe S1 bearer for the UE 4 are changed from the LPN 2 to the MeNB 1, thecontroller 15 may establish the S1 bearer and the DRB in the MeNB 1 byre-using the E-RAB configuration information, which has been kept in theMeNB 1, without re-sending the request for establishing the S1 bearer(or S1 handover request) to the MME 6.

Note that, when the endpoints of the S1 bearer and the DRB for the UE 4are changed from the LPN 2 to the MeNB 1 or another base station, thecontroller 15 may generate the DRB configuration information toestablish the DRB between the MeNB 1 or another base station and the UE4 based on the E-RAB configuration information kept in the MeNB 1. Thecontroller 15 may send the generated DRB configuration information tothe UE 4 via the SRB with the UE 4 on the cell 10.

Further, the controller 15 may notify the MME 6 of the endpointinformation of the S1 bearer changed from the LPN 2 to the MeNB 1 oranother base station. The MME 6 sends, to the S-GW 7, the receivedendpoint information of the S1 bearer, and the S-GW 7 updates the radioaccess network (RAN) side endpoint of the S1 bearer.

Further, the controller 15 may determine to switch the S1 bearer and theDRB for the UE 4 from the LPN 2 to the MeNB 1 or another base station,in response to a trigger notification from the UE 4 or the LPN 2.

FIG. 5 is a block diagram showing a configuration example of the LPN 2.The functions and the operations of a radio communication unit 21, atransmission data processing unit 22, a reception data processing unit23, and a communication unit 24 shown in FIG. 5 are similar to those ofthe corresponding elements of the base station 1 shown in FIG. 4, i.e.,the radio communication unit 11, the transmission data processing unit12, the reception data processing unit 13, and the communication unit14.

A controller 25 of the LPN 2 receives the E-RAB configurationinformation from the MeNB 1 (controller 15) via the X2/X3 signalingbearer, and sets up the S1 bearer with the S-GW 7 and the SRB with theUE 4 in accordance with the E-RAB configuration information.

FIG. 6 is a block diagram showing a configuration example of the UE 4. Aradio communication unit 41 can communicate with both the cell 10 andthe cell 20. In addition, the radio communication unit 41 may supportcarrier aggregation of a plurality of cells operated by different eNBs.In this case, the radio communication unit 41 can simultaneously use theplurality of cells 10 and 20 to transmit or receive user data. The radiocommunication unit 41 receives downlink signals from one or both of theeNB 1 and the LPN 2 via an antenna. A reception data processing unit 42restores received data from the received downlink signals, and sends thereceived data to a data controller 43. The data controller 43 uses thereceived data according to the purpose thereof. A transmission dataprocessing unit 44 and the radio communication unit 41 generate anuplink signal using transmission data supplied from the data controller43, and transmit the uplink signal to one or both of the eNB 1 and theLPN 2.

A controller 45 of the UE 4 controls the radio communication unit 41 toestablish the SRB with the MeNB 1 on the cell 10. The controller 45 thenreceives from the MeNB 1 the DBB configuration information to establishthe DRB with the LPN 2 and controls the radio communication unit 41 totransmit or receive the user data through the cell 20. Accordingly, theUE 4 can communicate with the LPN 2 via the DRB based on the signalingwith the MeNB 1.

Further, when the endpoints of the S1 bearer and the DRB for the UE 4are changed from the LPN 2 to the MeNB 1 or another base station, thecontroller 45 may receive, from the MeNB 1 through the SRB on the cell10, DRB configuration information to establish the DRB between the MeNB1 or another base station and the UE 4. Thus the UE 4 can change thedestination to which the DRB is connected from the LPN 2 to the MeNB 1or another base station based on the signaling with the MeNB 1.

FIG. 7 is a block diagram showing a configuration example of the MME 6.A communication unit 61 communicates with the MeNB 1 and the S-GW 7. Abearer setup controller 62 communicates with the MeNB 1 and the S-GW 7via the communication unit 51, and controls the setup of the data beareror the signaling bearer in these apparatuses. Specifically, in responseto receiving a setup request of the data bearer (E-RAB or S1 bearer)from the MeNB 1, the bearer setup controller 62 requests the S-GW 7 toset up the S1 bearer, and sends to the MeNB 1 the bearer configurationinformation (i.e., E-RAB configuration information) regarding the E-RABor the S1 bearer.

Further, the bearer setup controller 62 may receive, from the MeNB 1, amessage (e.g., path switch request) indicating that the endpoint of theS1 bearer which had been configured to the LPN 2 has been changed to theMeNB 1 or to another base station, and may instruct the S-GW 7 to changethe endpoint configuration of the S1 bearer in response to the message.

FIG. 8 is a block diagram showing a configuration example of the S-GW 7.A communication unit 71 establishes the S1 bearer with the LPN 2 andtransmits or receives user data to or from the LPN 2 through the S1bearer. The communication unit 71 may establish the S1 bearer with theMeNB 1 to receive or transmit the user data through the cell 10 by theUE 4. A communication unit 74 sets up an S5/S8 bearer with a Packet DataNetwork Gateway (P-GW) in the EPC 5 and transmits and receives user datato and from another data transfer apparatus.

A transmission data processing unit 72 receives downlink user datadestined for the UE 4 from the communication unit 74, and forwards thedownlink user data to the S1 bearer based on mapping between theupstream side S5/S8 bearer and the downstream side S1 bearer. Areception data processing unit 73 receives uplink user data from thecommunication unit 71 and forwards the uplink user data to the S5/S8bearer based on the mapping between the S5/S8 bearer and the S1 bearer.

A bearer controller 75 communicates with the MME 6 and sets up the S1bearer between the LPN 2 and the communication unit 71 in accordancewith the control of the MME 6. When the endpoint of the S1 bearer hasbeen changed from the LPN 2 to the MeNB 1 or another base station, thebearer controller 75 may change the RAN side endpoint configuration ofthe S1 bearer in accordance with the instruction from the MME 6.

In the following description, with reference to a sequence diagram inFIG. 9, specific examples of the communication control method accordingto the first embodiment will be described. In Step S101, the MeNB 1establishes the S1 connection associated with the UE 4 with the MME 6for the UE 4 which belongs to the cell 10. That is, the MeNB 1establishes the S1 signaling bearer with the MME 6 on the S1-MMEinterface. Further, the MeNB 1 establishes the RRC connection with theUE 4 on the cell 10. Accordingly, the control data is transferredbetween the UE 4 and the MeNB 1, between the MeNB 1 and the MME 6, andbetween the UE 4 and the MME 6.

In Steps S102 to S107, processing for establishing the S1 bearer and theDRB via the LPN 2 is performed. In Step S102, the MeNB 1 determines toset up the data bearer in the secondary cell (SCell). The MeNB 1 sendsto the MME 6 an establishment request (e.g., E-RAB SETUP message) of theE-RAB through the LPN 2 for the UE 4. Here, the secondary cell denotesthe cell 20 of the LPN 2. In other words, the MeNB 1 may determine toset up a secondary cell for the UE 4. For example the MeNB 1 maydetermine data bearer setup in the cell 20 in response to a request fromthe UE 4 or a request from the EPC 5. Alternatively, the MeNB 1 maydetermine data bearer setup in the cell 20 in response to a notificationfrom the UE 4 indicating that the cell 20 can be used. Alternatively,the MeNB 1 may determine data bearer setup in the cell 20 in response toan increase in the amount of user data of the UE 4 in the cell 10.Alternatively, when the cell 10 has a high load, the MeNB 1 maydetermine data bearer setup in the cell 20 to offload the traffic of thecell 10. Alternatively, the MeNB 1 may determine data bearer setup inthe cell 20 in accordance with subscriber data of the UE 4 (e.g.,category of the UE 4, contract information) received from a subscriberserver (i.e., Home Subscriber Server (HSS)) via the MME 6.

In response to the establishment request of the E-RAB from the MeNB 1,the MME 6 initiates setup procedure of the S1 bearer (Step S103). Morespecifically, the MME 6 requests the S-GW 7 to set up the S1 bearer withthe LPN 2. The S-GW 7 sets up the S1 bearer with the LPN 2 and sends tothe MME 6 a response including an S1 bearer context (e.g., a tunnelendpoint identifier (TEID) and an address of the S-GW 7 in the U-plane).The TEID indicates an S-GW 7 side endpoint of the GTP tunnel as the S1bearer. In Step S104, the MME 6 sends the E-RAB configurationinformation including the S1 bearer context to the MeNB 1. The E-RABconfiguration information is sent using, for example, an E-RAB SETUPRESPONSE message sent from the MME 6 to the MeNB 1.

In Step S105, the MeNB 1 sends the E-RAB configuration information tothe LPN 2 via the X2/X3 signaling bearer. The E-RAB configurationinformation includes S1 bearer configuration information and DRBconfiguration information. The LPN 2 sets up the S1 bearer and the DRBin accordance with the E-RAB configuration information. The S1 bearerconfiguration information includes information that is necessary toestablish the S1 bearer with the S-GW 7. The S1 bearer configurationinformation includes, for example, at least one of: an E-RAB ID; aQuality Class Indicator (QCI); the IP address of the S-GW 7; the S-GW 7side TEID of the GTP tunnel (S1 bearer); a security key; and a TemporaryMobile Subscriber Identity (TMSI) allocated to the UE 4. The DRBconfiguration information includes configuration information that isnecessary to establish the DRB with the UE 4. The DRB configurationinformation includes, for example, the E-RAB ID, the Quality ClassIndicator (QCI), and configuration information of the physical layer andthe MAC sublayer.

In Step S106, the MeNB 1 transmits, to the UE 4 on the SRB of the cell10, the configuration information of the DRB on the cell 20. Theconfiguration information of the DRB is transmitted using an RRCreconfiguration message. The UE 4 sets up the DRB in accordance with theconfiguration information of the DRB.

In Step S107, the MeNB 1 sends a message indicating the E-RAB setupcompletion (CREATE BEARER RESPONSE) to the MME 6. This message includesthe configuration information on the side of the LPN 2 regarding the S1bearer (e.g., the TEID and the address of the LPN 2). The MME 6 sends tothe S-GW 7 the message including the TEID and the address of the LPN 2.The S-GW 7 updates the S1 bearer configuration by the TEID and theaddress of the LPN 2 received from the MME 6.

According to the above processing of Steps S102 to S107, the E-RAB whichpasses through the LPN 2 is configured between the UE 4 and the S-GW 7.In Step S108, the UE 4 receives or transmits user data through the cell20 and the LPN 2.

In Step S109, the MeNB 1 keep the information regarding the LPN 2, i.e.,the configuration information of the E-RAB established in the LPN 2,without releasing this information even after the E-RAB has been set upin the LPN 2. The E-RAB configuration information kept in the MeNB 1includes the E-RAB configuration information received from the MME 6 inStep S104 to establish the E-RAB. The E-RAB configuration informationkept in the MeNB 1 includes, for example, at least one of: the E-RAB ID;the QCI; the IP address of the S-GW 7; the TEID of the GTP tunnel (S1bearer); the security key; and the TMSI allocated to the UE 4.

Second Embodiment

FIG. 10 shows a configuration example of a radio communication systemaccording to a second embodiment. This embodiment shows an example of amovement of the UE 4 within the cell 10 of the MeNB 1. Specifically, inthis embodiment, an example in which the cell 20 of the LPN 2 and a cell30 of an LPN 3 are closely deployed in the cell 10 so that the cell 20and the cell 30 partially overlap and the UE 4 moves from the cell 20 ofthe LPN 2 to the cell 30 of the LPN 3 will be described.

In this embodiment, when the endpoints of the S1 bearer and the DRB forthe UE 4 are changed from the LPN 2 to the LPN 3, the MeNB 1 sends tothe LPN 3 the E-RAB configuration information, which has been kept inthe MeNB 1. Thanks to re-use of the E-RAB configuration information thathas been kept in the MeNB 1, the MeNB 1 does not have to separatelysend, to the MME 6, an E-RAB establishment request or a handover requestto establish the E-RAB in the LPN 3. Accordingly, it is possible in thisembodiment to reduce the signaling with the MME 6 when the UE 4 movesbetween LPNs and to reduce a path switch delay (i.e., data bearerswitching delay time) when the UE 4 moves between LPNs.

FIG. 11 is a sequence diagram showing one example of the bearer switchprocedure according to the movement of the UE 4 within the cell 10. StepS201 corresponds to Step S109 shown in FIG. 9. That is, the MeNB 1 holdsthe E-RAB configuration information regarding the E-RAB which has beenestablished in the LPN 2. At this time, the UE 4 is located in the cell20 of the LPN 2. Accordingly, the UE 4 transmits and receives controldata through the cell 10 and the MeNB 1 (Step S202), and transmits andreceives user data through the cell 20 and the LPN 20 (Step S203).

In Step S204 or S205, the MeNB 1 receives a trigger notification (PATHSWITCH TRIGGER) from the LPN 2 or the UE 4. The trigger notificationincludes information for the MeNB 1 to determine path switch. The MeNB 1determines, based on the trigger notification from the LPN 2 or the UE4, switching of the data bearer route from the LPN 2 to the LPN 3 (i.e.,switching of the secondary cell (SCell)). Accordingly, in the exampleshown in FIG. 11, the LPN 2 is a source LPN and the LPN 3 is a targetLPN.

The trigger notification from the UE 4 to the MeNB 1 may be transmittedbased on radio quality of the LPN 2 measured by the UE 4 or may indicatethe radio quality of the LPN 2. The UE 4 may transmit the triggernotification when the radio quality of the LPN 2 is lower than apredetermined threshold. The trigger notification from the UE 4 to theMeNB 1 may be transmitted based on radio quality of another base station(e.g., LPN 3) measured by the UE 4 or may indicate it. The UE 4 maytransmit the trigger notification when the radio quality of another basestation (e.g., LPN 3) exceeds a predetermined threshold. The radioquality may be, for example, downlink received power, a Signal toInterference plus Noise Ratio (SINR), Received Signal Code Power (RSCP),or Reference Signal Received Quality (RSRQ).

On the other hand, the trigger notification from the LPN 2 to the MeNB 1may be sent based on load information indicating the load on the LPN 2or may indicate the load information. The LPN 2 may send the triggernotification when the load on the LPN 2 exceeds a predeterminedthreshold. The load on the LPN 2 may be, for example, a utilization rateof radio resources in the cell 20 (e.g., utilization rate of PhysicalResource Blocks (PRBs)). The trigger notification from the LPN 2 to theMeNB 1 may be sent based on a connection state of the UE 4 measured inthe LPN 2 or indicate the connection state. The LPN 2 may send thetrigger notification when the connection state of the UE 4 is moredegraded than a reference value. The connection state of the UE 4 maybe, for example, the number of occurrence or the rate of occurrence ofthe retransmission request from the UE 4 based on a Hybrid Automaticrepeat request (ARQ).

In Step S206, the MeNB 1 sends at least a part of the E-RABconfiguration information, which has been kept in the MeNB 1, to thetarget LPN 3. The target LPN 3 carries out the endpoint configuration ofthe S1 bearer and the DRB in the LPN 3 using the E-RAB configurationinformation received from the MeNB 1. The LPN 3 sends, to the MeNB 1,the LPN 3 side endpoint information of the S1 bearer for the UE 4. TheMeNB 1 updates the E-RAB configuration information, which has been keptin the MeNB 1, in order to reflect the LPN 3 side endpoint informationof the S1 bearer received from the LPN 3. In other words, the MeNB 1generates the E-RAB configuration information in which the data bearerconfiguration in the LPN 3 has been reflected and continues to hold thegenerated E-RAB configuration information.

In Step S207, the MeNB 1 generates, based on the E-RAB configurationinformation that has been kept in the MeNB 1, the DRB configurationinformation to establish the DRB between the UE 4 and the target LPN 3.The MeNB 1 transmits the DRB configuration information to the UE 4through the SRB on the cell 10. The UE 4 receives the DRB configurationinformation from the MeNB 1 and sets up the DRB on the cell 30.

Steps S208 to S210 are a procedure for switching the route of S1 bearer(i.e., procedure for switching the RAN side endpoint from the LPN 2 tothe LPN 3). In Step S208, the MeNB 1 sends to the MME 6 a message (PATHSWITCH REQUEST) requesting switching of the E-RAB. This message (PATHSWITCH REQUEST) indicates that the RAN side endpoint of the S1 bearerhas been changed from the LPN 2 to the LPN 3. This message (PATH SWITCHREQUEST) may include, for example, the E-RAB identifier (or S1 beareridentifier), an address of the LPN 3, and an endpoint identifier (TEID)of the S1 bearer in the LPN 3. In Step S209, the MME 6 sends a request(BEARER MODIFY) for updating the S1 bearer to the S-GW 7. The requestfor updating the S1 bearer includes, for example, the TEID and theaddress of the LPN 3. The S-GW 7 updates the S1 bearer configuration inaccordance with the TEID and the address of the LPN 3 received from theMME 6. In Step S210, the MME 6 sends to the MeNB 1 a message (PATHSWITCH RESPONSE) indicating the completion of switching the route of theS1 bearer.

According to the above processing of Steps 5210 and 5211, the E-RAB thatpasses through the LPN 3 is configured between the UE 4 and the S-GW 7.In Step S211, the UE 4 transmits and receives control data through thecell 10 and the MeNB 1 as similar to the case in which the UE 4communicates with the LPN 2 (Step S202). In Step S212, the UE 4 receivesor transmits user data through the cell 30 and the LPN 3.

In the following description, operations of the MeNB 1, the LPNs 2 and3, the UE 4, the MME 6, and the S-GW 7 according to the embodiment willbe described. FIG. 12 is a flowchart showing an operation example of theMeNB 1. In Step S301, the MeNB 1 (controller 15) determines whether toswitch the secondary cell of the UE 4 from the LPN 2 to another LPN (inthis example, to the LPN 3). When the switching of the secondary cell isdetermined (YES in Step S301), the MeNB 1 determines whether the MeNB 1already holds the E-RAB configuration information (Step S302). When theMeNB 1 does not hold the E-RAB configuration information (NO in StepS302), the MeNB 1 executes the normal bearer setup procedure (e.g.,handover procedure) and switches the data bearer for the UE 4 from theLPN 2 to the LPN 3 (Step S303). On the other hand, when the MeNB 1already holds the E-RAB configuration information (YES in Step S302),the MeNB 1 performs the processing of Step S304 and the followingprocessing.

In Step S304, the MeNB 1 sends to the target LPN 3 the E-RABconfiguration information based on the E-RAB configuration informationthat has been kept in the MeNB 1 (i.e., E-RAB configuration informationregarding the LPN 2). In Step S305, the MeNB 1 determines whether theE-RAB setup completion notification has been received from the targetLPN 3. When the E-RAB setup completion notification has been received(YES in Step S305), the MeNB 1 generates DRB configuration informationin which the DRB configuration in the LPN 3 has been reflected andtransmits the generated DRB configuration information to the UE 4 (StepS306). In Step S307, the MeNB 1 determines whether the DRB setupcompletion notification has been received from the UE 4. When the DRBsetup completion notification has been received (YES in Step S307), theMeNB 1 sends a path switch request (i.e., request for switching theroute of the S1 bearer) to the MME 6 (Step S308). When the path switchcompletion notification is received from the MME 6, the MeNB 1 completesthe processing of FIG. 12 (Step S309).

FIG. 13 is a flowchart showing an operation example of the LPNs 2 and 3.While the LPN 2 will be described in the following description, theoperation of the LPN 3 is similar to that of the LPN 2. In Step S401,the LPN 2 (controller 25) determines whether the E-RAB configurationinformation has been received from the MeNB 1. When the E-RABconfiguration information has been received (YES in Step S401), the LPN2 sets up the S1 bearer with the S-GW 7 and the DRB with the UE 4 inaccordance with the received E-RAB configuration information (Steps S402and S403). In Step S404, the LPN 2 notifies the MeNB 1 of the E-RABconfiguration completion.

FIG. 14 is a flowchart showing an operation example of the UE 4. In StepS501, the UE 4 (controller 45) receives the DRB configurationinformation from the MeNB 1. In Step S502, the UE 4 sets up the DRB(e.g., the DRB with the LPN 20 in the cell 20 or the DRB with the LPN 3in the cell 30) in accordance with the DRB configuration information.

FIG. 15 is a flowchart showing an operation example of the MME 6. InStep S601, the MME 6 (bearer setup controller 62) determines whether thepath switch request has been received from the MeNB 1. When the pathswitch request has been received (YES in Step S601), the MME 6 sends therequest for changing the route of the S1 bearer (i.e., a bearer updaterequest (BEARER MODIFY)) to the S-GW 7 (Step S602). In Step S603, theMME 6 determines whether the notification of the completion of thechange in the route of the S1 bearer has been received from the S-GW 7.When the completion notification has been received (YES in Step S603),the MME 6 notifies the MeNB 1 of the completion of the path switch(i.e., completion of the switch of the route of the S1 bearer) (StepS604).

FIG. 16 is a flowchart showing an operation example of the S-GW 7. InStep S701, the S-GW 7 (bearer controller 75) determines whether therequest for changing the route of the S1 bearer (i.e., a bearer updaterequest) has been received from the MME 6. The request for changing theroute of the S1 bearer indicates that the RAN side endpoint of the S1bearer for the UE 4 is changed from the LPN 2 to the LPN 3. When therequest for changing the route of the S1 bearer has been received (YESin Step S701), the S-GW 7 updates the configuration of the S1 bearer forthe UE 4 in accordance with the S1 bearer configuration information(Step S702). That is, the S-GW 7 changes the RAN side endpoint of the S1bearer for the UE 4 to the LPN 3. In Step S703, the S-GW 7 notifies theMME 6 of the completion of the route change (update completion) of theS1 bearer.

Third Embodiment

FIG. 17 shows a configuration example of a radio communication systemaccording to a third embodiment. This embodiment shows an example of themovement of the UE 4 within the cell 10 of the MeNB 1. Specifically, inthis embodiment, at least one LPN including the LPN 2 is sparselydeployed in the MeNB cell 10, and the LPN cell 20 does not overlapanother LPN cell (e.g., LPN cell 30). Accordingly, in this embodiment,an example in which the data bearer for the UE 4 is changed from a routethat passes through the LPN 2 to a route that passes through the MeNB 1will be described.

In this embodiment, when the endpoints of the S1 bearer and the

DRB for the UE 4 are changed from the LPN 2 to the MeNB 1, the MeNB 1establishes the S1 bearer and the DRB in the MeNB 1 by re-using theE-RAB configuration information that has been kept in the MeNB 1. Sincethe E-RAB configuration information that has been kept in the MeNB 1 isre-used, the MeNB 1 need not send to the MME 6 an E-RAB establishmentrequest or a handover request to establish the E-RAB in the MeNB 1. Itis thus possible in this embodiment to reduce signaling with the MME 6when the UE 4 moves from the LPN 2 to the MeNB 1 and to reduce a pathswitch delay when the UE 4 moves between cells (i.e., data bearerswitching delay time).

When the DRB for the UE 4 is established in the MeNB 1, this DRB may beconfigured on the cell 10 or may be configured on a cell (secondarycell) of the MeNB 1 different from the cell 10. When the DRB for the UE4 is configured on the cell 10, this cell operation mode is a typicalmode in which the same cell provides the C-Plane and the U-Plane. On theother hand, when the DRB for the UE 4 is configured on a cell of theMeNB 1 different from the cell 10, this cell operation mode correspondsto a so-called inter-base station carrier aggregation (Intra-eNB CarrierAggregation).

FIG. 18 is a sequence diagram showing one example of the bearer switchprocedure according to the movement of the UE 4 within the cell 10. Theprocessing in Steps S201 to S205 shown in FIG. 18 is similar to theprocessing in Steps S201 to S205 shown in FIG. 11. Specifically, theMeNB 1 determines, based on the trigger notification in Step S204 orS205, switching of the data bearer route from the LPN 2 to the MeNB 1.In Step S806, the MeNB 1 sets up the DRB and the S1 bearer for the UE 4in the MeNB 1 by re-using at least a part of the E-RAB configurationinformation that has been kept in the MeNB 1. The processing in StepsS207 to S211 shown in FIG. 18 is similar to that in Steps S207 to S211shown in FIG. 11. That is, the MeNB 1 transmits to the UE 4 the DRBconfiguration information to establish the DRB between the UE 4 and theMeNB 1. The MeNB 1 then requests the MME 6 to switch the route of the S1bearer. In Step S812 shown in FIG. 18, the UE 4 receives or transmitsuser data through the cell 10 or another cell of MeNB 1.

FIG. 19 is a flowchart showing an operation example of the

MeNB 1 according to the embodiment. In Step S901, the MeNB 1 (controller15) determines whether to switch the secondary cell for the UE 4 fromthe LPN 2 to the MeNB 1. The processing in Steps S302 and S303 shown inFIG. 19 is similar to that in Steps S302 and S303 shown in FIG. 12except that the secondary cell is switched to the MeNB 1 not to the LPN3. In Step S904, the MeNB 1 sets up the DRB and the S1 bearer for the UE4 in the MeNB 1 by re-using the E-RAB configuration information that hasbeen kept in the MeNB 1 (i.e., E-RAB configuration information regardingthe LPN 2). The processing in Steps S305 to S309 shown in FIG. 19 issimilar to that in Steps S305 to S309 shown in FIG. 12 except that thesecondary cell is switched to the MeNB 1 not to the LPN 3.

Fourth Embodiment

FIG. 20 shows a configuration example of a radio communication systemaccording to a fourth embodiment. This embodiment shows a modifiedexample of the second embodiment. Specifically, in this embodiment,there are candidate LPNs 3A and 3B to which the S1 bearer and the DRBfor the UE 4 established in the LPN 2 can be switched. The MeNB 1notifies the candidate LPNs 3A and 3B of the E-RAB configurationinformation preliminary to determining that the S1 bearer and the DRBfor the UE 4 should be changed from the LPN 2 to another LPN. The MeNB 1then determine a target LPN to which the data bearer is to be switched(e.g., the LPN 3A), and instruct the target LPN (LPN 3A) to activate thedata bearer. It is thus possible to enable the candidate LPNs to performat least a part of the configuration regarding the DRB and the S1 bearerfor the UE 4 in advance using the E-RAB configuration informationreceived from the MeNB 1. Accordingly, in this embodiment, it ispossible to further reduce the path switch delay (i.e., data bearerswitching delay time) when the UE 4 moves between cells.

FIG. 21 is a sequence diagram showing one example of the bearer switchprocedure according to the movement of the UE 4 within the cell 10. Theprocessing in Steps S201 to S203 shown in FIG. 21 is similar to theprocessing in Steps S201 to S203 shown in FIG. 11. In Step S1004 shownin FIG. 21, the MeNB 1 sends the E-RAB configuration information, whichhas been kept in the MeNB 1, to the plurality of candidate LPNs 3A and3B. The processing in Step S1005 is similar to that in Steps S204 andS205 shown in FIG. 11. Specifically, the MeNB 1 receives the triggernotification from the LPN 2 or the UE 4.

In Step S1006, the MeNB 1 determines the target LPN from the pluralityof candidate LPNs 3. The target LPN is the destination to which theendpoints of the DRB and the S1 bearer for the UE 4 that has beenestablished in the LPN 2 (source LPN) are to be switched. The MeNB 1 mayselect, as the target LPN, a candidate LPN that satisfies apredetermined condition from the plurality of candidate LPNs. Thepredetermined condition relates to, for example, at least one of (a)radio quality of each candidate LPN measured by the UE 4, (b) load oneach candidate LPN, and (c) moving speed of the UE 4. The MeNB 1 mayreceive the radio quality information of each candidate LPN or loadinformation of each candidate LPN from each candidate base station. TheMeNB 1 may receive radio quality information of each candidate LPN fromthe UE 4. In some implementation, the MeNB 1 may collect loadinformation of each candidate LPN and select, as the target LPN, acandidate LPN whose load is below a predetermined threshold.Alternatively, the MeNB 1 may collect radio quality information of eachcandidate LPN and select, as the target LPN, a candidate LPN whose radioquality exceeds a predetermined threshold.

In Step S1007, the MeNB 1 sends the bearer activation information (E-RABACTIVATION) to the target LPN selected from the plurality of candidateLPNs. In the example shown in FIG. 21, the LPN 3A is selected as thetarget LPN. In response to receiving the bearer activation information,the LPN 3A sets up the DRB for the UE 4 in the cell 30A. The processingin S207 to S211 shown in FIG. 21 is similar to the processing in StepsS207 to S211 shown in FIG. 11.

FIG. 22 is a flowchart showing an operation example of the MeNB 1according to this embodiment. In Step S1101, the MeNB 1 (controller 15)preliminarily sends, to the candidate LPN, the E-RAB configurationinformation regarding the E-RAB which has been configured in the LPN 2.The processing in Steps S301 to S303 shown in FIG. 22 is similar to theprocessing in Steps S301 to S303 shown in FIG. 12. In Step S1104, theMeNB 1 selects the target LPN from the plurality of candidate LPNs. InStep S1105, the MeNB 1 sends E-RAB activation information to the targetLPN. The processing in Steps S306 to S309 shown in FIG. 22 is similar tothe processing in Steps S306 to S309 shown in FIG. 12.

FIG. 23 is a flowchart showing an operation example of the candidateLPN, i.e., the LPN 3A (or 3B), according to this embodiment. In StepS1201, the LPN 3A receives the E-RAB configuration information from theMeNB 1 in advance and sets up the S1 bearer with the S-GW 7 in advance.In Step S1202, the LPN 3A determines whether the E-RAB activationinformation has been received from the MeNB 1. The processing in StepsS403 and S404 shown in FIG. 23 is similar to the processing in StepsS403 and S404 shown in FIG. 13. That is, when the E-RAB activationinformation has been received (YES in Step S1202), the LPN 3A sets upthe DRB with the UE 4 on the cell 30A (Step S403). The LPN 3A notifiesthe MeNB 1 of the E-RAB setup completion.

Other Embodiments

The above first to fourth embodiments may be appropriately combined. Forexample, the second and third embodiments may be combined with eachother or the third and fourth embodiments may be combined with eachother. In this case, the MeNB 1 may determine whether the endpoints ofthe DRB and the S1 bearer for the UE 4 are changed to the MeNB 1 or tothe LPN 3, based on the moving speed of the UE 4 or the frequency of themovement between cells. For example, when the moving speed or frequencyof inter-cell movement of the UE 4 exceeds a predetermined threshold,the MeNB 1 may select the MeNB 1 as the destination to which the bearerendpoints for the UE 4 to be changed. It is thus possible to suppressfrequent occurrence of processing for changing the bearer route whichoccurs due to a frequent movement of the UE 4 between LPNs. On the otherhand, when the moving speed or frequency of inter-cell movement of theUE 4 is below a predetermined threshold, the MeNB 1 may select the LPN 3as the destination to which the bearer endpoints for the UE 4 to bechanged.

All the communication control methods in the C/U-plane split scenario bythe MeNB 1, the LPN 2, the LPN 3, the UE 4, the MME 6, and the S-GW 7described in the first to fourth embodiments may be implemented by usinga semiconductor processing device including an Application SpecificIntegrated Circuit (ASIC). Alternatively, these methods may beimplemented by causing a computer system including at least oneprocessor (e.g., microprocessor, Micro Processing Unit (MPU), DigitalSignal Processor (DSP)) to execute a program. Specifically, one or moreprograms including instructions for causing a computer system to performthe algorithms shown in the flowcharts and the sequence diagrams may becreated and these programs may be supplied to a computer.

These programs can be stored and provided to a computer using any typeof non-transitory computer readable media. Non-transitory computerreadable media include any type of tangible storage media. Examples ofnon-transitory computer readable media include magnetic storage media(such as flexible disks, magnetic tapes, hard disk drives, etc.),optical magnetic storage media (e.g., magneto-optical disks), CompactDisc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories(such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flashROM, Random Access Memory (RAM), etc.). These programs may be providedto a computer using any type of transitory computer readable media.Examples of transitory computer readable media include electric signals,optical signals, and electromagnetic waves. Transitory computer readablemedia can provide a program to a computer via a wired communication line(e.g., electric wires, and optical fibers) or a wireless communicationline.

In the above first to fourth embodiments, the LTE system has been mainlydescribed. However, these embodiments may be applied to radiocommunication systems other than the LTE system, for example, a 3GPPUniversal Mobile Telecommunications System (UMTS), a 3GPP2 CDMA2000system (1xRTT, High Rate Packet Data (HRPD)), a Global System for MobileCommunications (GSM) system, or a WiMAX system.

Further, the above embodiments are merely examples of applications oftechnical ideas obtained by the present inventors. Needless to say,these technical ideas are not limited to the above embodiments and maybe changed in various ways.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-288209, filed on Dec. 28, 2012, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   1 Base Station (MenB)-   2 Base Station (LPN)-   3, 3 a, 3 b Base Stations (LPNs)-   4 Mobile Station (UE)-   5 Core Network (EPC)-   6 Mobility Management Apparatus (MME)-   7 Data Transfer Apparatus (S-GW)-   15 Controller-   25 Controller-   45 Controller-   62 Bearer Setup Controller-   75 Bearer Controller-   80 Tunnel

The invention claimed is:
 1. A radio communication system comprising: afirst base station that operates a first cell; a second base stationthat operates a second cell; a core network comprising a mobilitymanagement apparatus and a data transfer apparatus; and a mobilestation, wherein the first base station is configured to establish afirst signaling bearer with the mobility management apparatus, establisha second signaling bearer with the second base station, and establish asignaling radio bearer with the mobile station in the first cell, thesecond base station is configured to establish the second signalingbearer with the first base station, establish a data bearer with thedata transfer apparatus, and establish a data radio bearer with themobile station in the second cell, the first base station is furtherconfigured to: send, to the second base station via the second signalingbearer, first configuration information for establishing the data bearerand the data radio bearer in the second base station, keep the firstconfiguration information in the first base station even after the databearer and the data radio bearer are established in the second basestation; and when an endpoint of the data radio bearer and an endpointof the data bearer are changed from the second base station to a thirdbase station: send the first configuration information, which has beenkept in the first base station, to the third base station, and send amessage including an address and a Tunnel Endpoint Identifier (TEID) ofthe third base station to the mobility management apparatus.
 2. Theradio communication system according to claim 1, wherein the first basestation is configured to: when the data radio bearer and the data bearerare established in the second base station, receive from the mobilitymanagement apparatus the first configuration information as a responseto a request for establishing the data bearer sent from the first basestation to the mobility management apparatus, and when the endpoint ofthe data radio bearer and the endpoint of the data bearer are changedfrom the second base station to the third base station, send the firstconfiguration information, which has been kept in the first basestation, to the third base station without sending a request forestablishing the data bearer to the mobility management apparatus. 3.The radio communication system according to claim 1, wherein the firstbase station is configured to notify the mobility management apparatusof endpoint information of the data bearer whose endpoint has beenchanged to the third base station.
 4. The radio communication systemaccording to claim 1, wherein the first base station is configured tokeep second configuration information generated by updating the firstconfiguration information using information of the third base stationeven after the endpoint of the data radio bearer and the endpoint of thedata bearer have been changed to the third base station.
 5. The radiocommunication system according to claim 1, wherein the first basestation is configured to receive, from the third base station, endpointinformation of the data bearer whose endpoint has been changed to thethird base station.
 6. The radio communication system according to claim1, wherein the first base station is configured to, when the endpoint ofthe data radio bearer and the endpoint of the data bearer are changedfrom the second base station to the third base station, generate, basedon the first configuration information, third configuration informationto establish the data radio bearer between the mobile station and thethird base station and transmits the third configuration information tothe mobile station via the signaling radio bearer.
 7. The radiocommunication system according to claim 1, wherein the first basestation is configured to send the first configuration information to aplurality of candidate base stations including the third base stationbefore the third base station is selected as a base station to which thedata bearer and the data radio bearer are switched, and the first basestation is configured to instruct the third base station to activate thedata bearer after the third base station is selected.
 8. The radiocommunication system according to claim 7, wherein the first basestation is configured to select, as the third base station, a candidatebase station which satisfies a predetermined condition from theplurality of candidate base stations.
 9. The radio communication systemaccording to claim 8, wherein the predetermined condition relates to atleast one of: (a) radio quality of each candidate base station measuredby the mobile station; (b) load on each candidate base station; and (c)moving speed of the mobile station.
 10. The radio communication systemaccording to claim 9, wherein the first base station is configured toreceive at least one of the radio quality and the load from eachcandidate base station.
 11. The radio communication system according toclaim 1, wherein the first base station is configured to, when theendpoint of the data radio bearer and the endpoint of the data bearerare changed from the second base station to the first base station,establish the data bearer and the data radio bearer in the first basestation by re-using the first configuration information that has beenkept in the first base station.
 12. The radio communication systemaccording to claim 11, wherein the first base station is configured to,when the data radio bearer and the data bearer are established in thesecond base station, receive from the mobility management apparatus thefirst configuration information as a response to a request forestablishing the data bearer sent from the first base station to themobility management apparatus, and the first base station is configuredto, when the endpoint of the data radio bearer and the data bearer ischanged from the second base station to the first base station,establish the data bearer and the data radio bearer in the first basestation by using the first configuration information, which has beenkept in the first base station, without sending a request forestablishing the data bearer to the mobility management apparatus. 13.The radio communication system according to claim 11, wherein the firstbase station is configured to determine, based on a moving speed of themobile station, whether the endpoint of the data radio bearer and theendpoint of the data bearer are changed to the first base station oranother base station.
 14. The radio communication system according toclaim 1, wherein the first base station is configured to determine tochange the endpoint of the data radio bearer and the endpoint of thedata bearer, in response to a trigger notification from the second basestation or the mobile station.
 15. The radio communication systemaccording to claim 1, wherein the mobile station is configured totransmit or receive user data via the data radio bearer withoutestablishing a signaling radio bearer with the second base station. 16.The radio communication system according to claim 1, wherein anon-access stratum control message is transferred between the corenetwork and the mobile station via the first signaling bearer and thesignaling radio bearer.
 17. The radio communication system according toclaim 1, wherein the first configuration information comprises at leastone of an identifier of the data bearer, QoS information of the databearer, an address of the data transfer apparatus, a tunnel endpointidentifier of the data transfer apparatus, and an identifier of themobile station.
 18. A first base station comprising: a controllerconfigured to: perform control to establish a first signaling bearerwith a mobility management apparatus in a core network, establish asecond signaling bearer with a second base station that operates asecond cell, and establish a signaling radio bearer with a mobilestation in the first cell; send, to the second base station via thesecond signaling bearer, first configuration information forestablishing a data bearer and a data radio bearer in the second basestation, wherein the data bearer is established between the second basestation and a data transfer apparatus in the core network, and the dataradio bearer is established between the second base station and themobile station in the second cell; keep the first configurationinformation even after the data bearer and the data radio bearer areestablished in the second base station; when an endpoint of the dataradio bearer and an endpoint of the data bearer are changed from thesecond base station to a third base station: send the firstconfiguration information, which has been kept in the first basestation, to the third base station, and send a message including anaddress and a Tunnel Endpoint Identifier (TEID) of the third basestation to the mobility management apparatus.
 19. The first base stationaccording to claim 18, wherein the controller is configured to: when thedata radio bearer and the data bearer are established in the second basestation, receive from the mobility management apparatus the firstconfiguration information as a response to a request for establishingthe data bearer sent from the first base station to the mobilitymanagement apparatus, and when the endpoint of the data radio bearer andthe data bearer is changed from the second base station to the thirdbase station, send the first configuration information, which has beenkept in the first base station, to the third base station withoutsending a request for establishing the data bearer to the mobilitymanagement apparatus.
 20. The first base station according to claim 18,wherein the controller is configured to notify the mobility managementapparatus of endpoint information of the data bearer whose endpoint hasbeen changed to the third base station.
 21. The first base stationaccording to claim 18, wherein the controller is configured to keepsecond configuration information generated by updating the firstconfiguration information using information of the third base stationeven after the endpoint of the data radio bearer and the endpoint of thedata bearer have been changed to the third base station.
 22. The firstbase station according to claim 18, wherein the controller is configuredto receive, from the third base station, endpoint information of thedata bearer whose endpoint has been changed to the third base station.23. The first base station according to claim 18, wherein the controlleris configured to, when the endpoint of the data radio bearer and theendpoint of the data bearer are changed from the second base station tothe third base station, generate, based on the first configurationinformation, third configuration information to establish the data radiobearer between the mobile station and the third base station andtransmits the third configuration information to the mobile station viathe signaling radio bearer.
 24. The first base station according toclaim 18, wherein the controller is configured to send the firstconfiguration information to a plurality of candidate base stationsincluding the third base station before the third base station isselected as a base station to which the data bearer and the data radiobearer are changed, and the controller is configured to instruct thethird base station to activate the data bearer after the third basestation is selected.
 25. The first base station according to claim 24,wherein the controller is configured to select, as the third basestation, a candidate base station which satisfies a predeterminedcondition from the plurality of candidate base stations.
 26. The firstbase station according to claim 25, wherein the predetermined conditionrelates to at least one of: (a) radio quality of each candidate basestation measured by the mobile station; (b) load on each candidate basestation; and (c) moving speed of the mobile station.
 27. The first basestation according to claim 26, wherein the controller is configured toreceive at least one of the radio quality and the load from eachcandidate base station.
 28. The first base station according to claim18, wherein the controller is configured to, when the endpoint of thedata radio bearer and the data bearer is changed from the second basestation to the first base station, establish the data bearer and thedata radio bearer in the first base station by using the firstconfiguration information that has been kept in the first base station.29. The first base station according to claim 28, wherein the controlleris configured to: when the data radio bearer and the data bearer areestablished in the second base station, receive, from the mobilitymanagement apparatus, the first configuration information as a responseto a request for establishing the data bearer sent from the first basestation to the mobility management apparatus, and when the endpoint ofthe data radio bearer and the endpoint of the data bearer are changedfrom the second base station to the first base station, establish thedata bearer and the data radio bearer in the first base station by usingthe first configuration information, which has been kept in the firstbase station, without sending a request for establishing the data bearerto the mobility management apparatus.
 30. The first base stationaccording to claim 28, wherein the controller is configured todetermine, based on a moving speed of the mobile station, whether theendpoint of the data radio bearer and the endpoint of the data bearerare changed to the first base station or another base station.
 31. Thefirst base station according to claim 18, wherein the controller isconfigured to determine to change the endpoint of the data radio bearerand the endpoint of the data bearer, in response to a triggernotification from the second base station or the mobile station.
 32. Amobile station that is used in combination with the radio communicationsystem according to claim 1, the mobile station comprising: a radioreceiver; and a controller configured to control the radio receiver toreceive configuration information regarding the data radio bearer fromthe first base station and receive or transmit user data using thesecond cell.
 33. The mobile station according to claim 32, wherein thecontroller is configured to control transmission or reception of theuser data via the data radio bearer without establishing a signalingradio bearer with the second base station.
 34. A communication controlmethod in a first base station that operates a first cell, thecommunication control method comprising: performing control to establisha first signaling bearer with a mobility management apparatus in a corenetwork, establish a second signaling bearer with a second base stationthat operates a second cell, and establish a signaling radio bearer witha mobile station in the first cell; sending, to the second base stationvia the second signaling bearer, first configuration information forestablishing a data bearer and a data radio bearer in the second basestation, wherein the data bearer is established between the second basestation and a data transfer apparatus in the core network, and the dataradio bearer is established between the second base station and themobile station in the second cell; keeping the first configurationinformation in the first base station even after the data radio bearerand the data bearer are established in the second base station; and whenan endpoint of the data radio bearer and an endpoint of the data bearerare changed from the second base station to a third base station:sending the first configuration information, which has been kept in thefirst base station, to the third base station, and sending a messageincluding an address and a Tunnel Endpoint Identifier (TEID) of thethird base station to the mobility management apparatus.
 35. The methodaccording to claim 34, further comprising, when the data radio bearerand the data bearer are established in the second base station,receiving from the mobility management apparatus the first configurationinformation as a response to a request for establishing the data bearersent from the first base station to the mobility management apparatus,wherein the sending the first configuration information to the thirdbase station includes sending the first configuration information, whichhas been kept in the first base station, to the third base stationwithout sending a request for establishing the data bearer to themobility management apparatus.
 36. The method according to claim 34,further comprising notifying the mobility management apparatus ofendpoint information of the data bearer whose endpoint has been changedto the third base station.
 37. The method according to claim 34, whereinthe sending the first configuration information to the third basestation includes: sending the first configuration information to aplurality of candidate base stations including the third base stationbefore the third base station is selected as a base station to which thedata bearer and the data radio bearer are changed; and instructing thethird base station to activate the data bearer after the third basestation is selected.
 38. The method according to claim 37, wherein thesending the first configuration information to the third base stationincludes selecting, as the third base station, a candidate base stationwhich satisfies a predetermined condition from the plurality ofcandidate base stations.
 39. The method according to claim 34, furthercomprising, when the endpoint of the data radio bearer and the endpointof the data bearer are changed from the second base station to the firstbase station, establishing the data bearer and the data radio bearer inthe first base station by using the first configuration information thathas been kept in the first base station.
 40. The method according toclaim 39, further comprising, when the data radio bearer and the databearer are established in the second base station, receiving, from themobility management apparatus, the first configuration information as aresponse to a request for establishing the data bearer sent from thefirst base station to the mobility management apparatus, wherein theestablishing the data bearer and the data radio bearer in the first basestation includes configuring the data bearer and the data radio bearerin the first base station by using the first configuration information,which has been kept in the first base station, without sending a requestfor establishing the data bearer to the mobility management apparatus.41. The method according to claim 39, further comprising determining,based on a moving speed of the mobile station, whether the endpoint ofthe data radio bearer and the endpoint of the data bearer are changed tothe first base station or another base station.
 42. A non-transitorycomputer readable medium storing a program for causing a computer toperform a communication control method in a first base station thatoperates a first cell, wherein the communication control methodcomprises: performing control to establish a first signaling bearer witha mobility management apparatus in a core network, establish a secondsignaling bearer with a second base station that operates a second cell,and establish a signaling radio bearer with a mobile station in thefirst cell, sending, to the second base station via the second signalingbearer, first configuration information for establishing a data bearerand a data radio bearer in the second base station, wherein the databearer is established between the second base station and a datatransfer apparatus in the core network, and the data radio bearer isestablished between the second base station and the mobile station inthe second cell; keeping the first configuration information in thefirst base station even after the data radio bearer and the data bearerare established in the second base station; and when an endpoint of thedata radio bearer and an endpoint of the data bearer are changed fromthe second base station to a third base station: sending the firstconfiguration information, which has been kept in the first basestation, to the third base station, and sending a message including anaddress and a Tunnel Endpoint Identifier (TEID) of the third basestation to the mobility management apparatus.
 43. A master base stationcomprising: a memory storing instructions; and at least one hardwareprocessor configured to process the instructions to: transmit a firstmessage, from the master base station to a target base station, fortransferring an user equipment (UE) from a source base station to thetarget base station wherein the first message includes an identifier ofan E-UTRAN Radio Access Bearer (E-RAB) of the source base station, aquality parameter of the E-RAB, and S1 General Packet Radio ServiceTunneling Protocol (GTP) Tunnel Endpoint information corresponding tothe E-RAB; receive, from the target base station, a response of thefirst message, the response including a Tunnel Endpoint Identifier(TEID) of the target base station corresponding to the E-RAB; andtransmit, to a Mobility Management Entity (MME), a second messageincluding an address of the target base station and the TEID of thetarget base station corresponding to the E-RAB.
 44. The master basestation according to claim 43, wherein an S1-MME interface for the UE isterminated in the master base station, and S1-U interfaces for the UEare terminated in the source base station and the target base station.45. The master base station according to claim 44, wherein the S1-MMEinterface is used for Control Plane, and the S1-U interfaces are is usedfor User Plane.
 46. The master base station according to claim 44,wherein the S1-MME interface and the S1-U interfaces are usedsimultaneously.
 47. A communication control method in a master basestation, the method comprising: transmitting a first message, to atarget base station, for transferring a user equipment (UE) from asource base station to the target base station, wherein the firstmessage includes an identifier of an E-UTRAN Radio Access Bearer (E-RAB)of the source base station, a quality parameter of the E-RAB, and S1General Packet Radio Service Tunneling Protocol (GTP) Tunnel Endpointinformation corresponding to the E-RAB; receiving, from the target basestation, a response of the first message, the response including aTunnel Endpoint Identifier (TEID) of the target base stationcorresponding to the E-RAB; and transmitting, to a Mobility ManagementEntity (MME), a second message including an address of the target basestation and the TEID of the target base station.
 48. The methodaccording to the claim 47, wherein an S1-MME interface for the UE isterminated in the master base station, and S1-U interfaces for the UEare terminated in the source base station and the target base station.49. The method according to the claim 48, wherein the S1-MME interfaceis used for Control Plane, and the S1-U interfaces are is used for UserPlane.
 50. The method according to the claim 48, wherein the S1-MMEinterface and the S1-U interfaces are used simultaneously.